In the process of the present invention aromatics are formed from feed hydrocarbons by dehydrocyclization. Other reactions may take place in the reaction zone, and the reaction step may more generally be referred to as reforming. Thus, in the reaction step of the process of the present invention, besides the dehydrocyclization or aromatization reaction, other reactions can occur, such as dehydrogenation, isomerization, hydroisomerization, cyclization and hydrocracking. The main reaction is dehydrocyclization to form aromatics from paraffins.
U.S. Pat. No. 4,104,320, which was granted Aug. 1, 1978, discloses that it is possible to dehydrocyclize paraffins to produce aromatics with high selectivity using a monofunctional nonacidic catalyst. Preferably, the catalyst comprises a type L zeolite. The type L zeolite preferred in the U.S. Pat. No. 4,104,320 process are those having exchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium. The catalyst used in U.S. Pat. No. 4,104,320 also contains at least one Group VIII noble metal (or tin or germanium). In particular, catalysts having platinum in an L zeolite, wherein potassium in the L zeolite has been exchanged to replace a portion of the potassium with rubidium or cesium, are claimed in the '320 patent to achieve exceptionally high selectivity for n-hexane conversion to benzene. As disclosed in U.S. Pat. No. 4,104,320, the L zeolite used is typically synthesized in the potassium form. A portion, usually not more than 80%, of the potassium cations can be exchanged so that other cations, preferably rubidium or cesium, replace the exchangeable potassium.
After U.S. Pat. No. 4,104,320 in 1978, an important step forward was disclosed in U.S. Pat. Nos. 4,434,311; 4,435,283; 4,447,316; and 4,517,306, all of which patents were granted in 1984 and 1985. These patents describe dehydrocyclization catalysts comprising a large-pore zeolite exchanged with an alkaline earth metal (barium, strontium or calcium, preferably barium) and wherein the catalyst contains one or more Group VIII metals, most preferably platinum. An essential element in the catalyst of these patents is the alkaline earth metal. Especially when the alkaline earth metal is barium, and the large-pore zeolite is L zeolite, the catalysts of these patents were found to provide higher selectivities than the corresponding alkali exchanged L zeolite catalyst disclosed in U.S. Pat. No. 4,104,320.
These platinum on L zeolite catalysts referred to in the previous two paragraphs, whether in the potassium form, or other alkali metal form, or in the alkaline earth metal exchanged form, are substantially "nonacidic". These nonacidic catalysts have been referred to as "monofunctional" catalysts. Such nonacidic, monofunctional catalysts are highly selective for forming aromatics via dehydrocyclization of paraffins.
Having a highly selective catalyst, commercialization seemed straightforward. However, that was not the case. It was found that the high selectivity L zeolite catalysts containing a Group VIII metal were unexpectedly susceptible to sulfur poisoning at ultra low levels of sulfur in the feed. U.S. Pat. No. 4,456,527 discloses this discovery. Specifically, it was found that the concentration of sulfur in the hydrocarbon feed must be reduced to ultra low levels, preferably less than 50 parts per billion, to achieve acceptable stability, i.e., long run length, for the catalyst when used in the dehydrocyclization process.
With the progress of U.S. Pat. No. 4,456,527, more attention was given to the process arrangement under which the overall dehydrocyclization process to produce aromatics was carried out.
U.S. Pat. Nos. 4,648,961 and 4,650,565 disclose processes using a highly selective dehydrocyclization catalyst wherein a paraffins rich feed is contacted with the catalyst to form aromatics, then the aromatics are separated from the reaction zone effluent by means of a solvent extraction step, or via molecular sieve separation, and a raffinate paraffins rich stream from the solvent extraction or molecular sieve separation step is recycled to the dehydrocyclization reaction zone.
U.S. Pat. No. 4,594,145 and Reissue Pat. No. 33,323 disclose a process using a highly selective dehydrocyclization catalyst wherein a paraffins rich feed is contacted with the catalyst to form aromatics, then aromatics are separated from the reaction zone effluent, and the remaining paraffins are recycled to the reaction zone.
U.S. Pat. Nos. 4,568,656 and 4,595,668 disclose use of highly selective dehydrocyclization catalyst wherein the Group VIII metal component of the catalyst is highly dispersed on a zeolite L support. These two U.S. patents state (see Col. 16, line 38, of the '668 patent): "Since the catalyst is monofunctional and does not promote isomerization without cyclization, feed compounds such as dimethylbutanes are not effective."
Another process oriented patent using a highly selective dehydrocyclization catalyst is European Patent 335,540. This patent discloses a process wherein a hydrocarbon feed is (a) separated into a first fraction comprising C.sub.5 minus hydrocarbons and dimethylbutanes, and a second fraction comprising C.sub.6 plus hydrocarbons; (b) separating the second fraction into (i) a light fraction comprising not more than 10% by volume dimethylbutanes, the light fraction being selected from a C.sub.6 fraction, a C.sub.7, a C.sub.8 fraction, a C.sub.6 -C.sub.7 fraction, C.sub.7 -C.sub.8 fraction, C.sub.6 -C.sub.8 and a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbon; and (ii) a heavy fraction; and (c) dehydrocyclizing the light fraction under dehydrocyclization conditions in the presence of a monofunctional catalyst. Thus, according the EP 335,540 process, dimethylbutanes are removed from the hydrocarbon fresh feed prior to dehydrocyclization of the fresh feed to form aromatics.